Data Communication
Prof. Ajit Pal
Department of Computer Science & Engineering
Indian Institute of Technology, Kharagpur
Lecture No # 7
Transmission of Digital Signal-I
Hello and welcome to today’s lecture. In the last couple of lectures we have discussed
about various transmission media.
Now we shall focus on a signal that we shall transmit through the communication media.
And today we shall discus about transmission of digital signal and it is the first lecture on
the transmission of digital signal.
(Refer Slide Time: 01:21)
So, topic is transmission of digital signal minus I. here is the outline of the lecture. First I
shall give a brief introduction then we shall discuss the important characteristics of line
coding then popular line coding techniques such as unipolar, polar and bipolar then we
shall consider a very important characteristic that is the modulation rate of various codes
that we shall discuss in this lecture then we shall compare the line coding techniques that
we shall cover in this lecture.
(Refer Slide Time: 01:58)
And obviously on completion the students will be able to explain the need for digital
transmission, why digital transmission is required. They will be able to explain the basic
concepts of line coding, explain the important characteristics of line coding, they will be
able to distinguish among various line coding techniques such as unipolar, polar and
bipolar and they will be able to distinguish between the data rate and modulation rate.
(Refer Slide Time: 02:15)
Before I discuss about the transmission of digital signal let us very quickly give an
overview of the transmission media that we discussed in the last two lectures.
(Refer Slide Time: 03:23)
As you remember we discussed about two types of transmission media; the guided media
and unguided media. The first few here for example UTP, coaxial cable, optical fiber
these three belong to the guided transmission media. On the other hand radio, microwave,
satellite and infrared belong to unguided transmission media. And here the first column
gives you the cost comparison. obviously the UTP is of lowest cost, coaxial has moderate
cost, optical fiber has got high cost, radio communication has got moderate cost,
microwave has got high cost, satellite has got high cost and infrared has got low cost.
In terms of bandwidth as you know UTP has got lowest bandwidth and optical fiber has
got the highest bandwidth then so far as the unguided media is concerned microwave
gives you the highest bandwidth and of course microwave is used in two cases. The first
one essentially terrestrial microwave and second one is satellite microwave which gives
you high bandwidth.
In terms of attenuation we find optical fiber attenuation is the lowest and of course for
microwave as you can see the attenuation is variable. Depending on the atmospheric
condition it will vary.
Electromagnetic interference is high in the first two cases UTP and coaxial cable. in case
of optical fiber it is minimum as you can see here, then for radio that electromagnetic
interference is very high, for microwave and satellite these are also high. for infrared it is
low.
And so far as security is concerned the UTP and coaxial cable is not at all secured they
are prone to …….. dropping and on the other hand optical fiber has got very high security
(()) is very very difficult and on the other hand radio wave since it is broadcasting nature
it has got very low security, microwave and satellite has got moderate security because it
is not that easy to receive the signal from terrestrial microwave or satellite where you
have to set up an antenna which will require good amount of effort. On the other hand
infrared has got high security because it is within a room.
So with this background now we shall discuss the transmission of digital signal.
But before doing that let us consider the various options available to you. As you know
you can have digital data and you can get it converted into digital signal by suitable
encoding technique similarly you can have analogue data which can also be converted
into digital signal by suitable encoding then you can have analog data which can be
converted into analog signal by suitable modulation technique. We shall discuss this later
in this lecture then we can have digital data which can be converted into analog signal by
suitable modulation techniques. So these are the various options available to us.
(Refer Slide Time: 06:30)
The question arises as what type of signal we should use? Is it digital signal or analog
signal? As we shall see each of these signals has its own characteristics and features,
good points and bad points and as we know a digital signal requires a low pass channel or
transmission media and bandwidth requirement is more. on the other hand analog signal
requires a bandpass communication media or bandpass channel. And most of the
practical channels that we shall encounter are bandpass in nature as we shall see.
So depending on the situation and the bandwidth availability we shall choose either
digital signal or analog signal. So in some situation we shall use digital signal and in
some other situation we shall go for analog signal so we shall use both.
So in this lecture we shall start with the digital data and digital signal. And as you know
the bandwidth of the medium will play a very crucial role when we send through
transmission media. We have to match the bandwidth of the signal with the bandwidth of
the transmission media so that the signal can pass efficiently without much attenuation,
without much distortion and this will require a good encoding technique which we shall
discuss in this lecture. So in this lecture we shall consider digital data which is converted
into digital signal and the technique that is being used is known as line coding. Line
coding technique is used for converting digital data into digital signal. This can be
explained with the help of the simple diagram.
(Refer Slide Time: 09:03)
here you have got the digital data 1000001 this data may be coming from some memory
of the computer or may be stored in the digital disk and that we shall apply to a system or
we shall do line coding which will convert it into a signal as you can see here, here you
have got the signal. This signal can be transmitted through the communication media.
Line coding is the technique that we shall discuss today.
What are the important characteristics that we have to consider for line coding?
Here are some of the important characteristics are given. First one is the number of signal
levels. As we do the encoding then the number of signal levels can remain 2 because it is
binary signal binary data it can be coded with two levels or you can do multilevel
encoding. for example as it is shown here your data has got two levels 0 and 1 and the
signal has also got two levels so two signal levels one is 0V and the other is 2A volt that
means 2A is the amplitude and 0 is the amplitude. So it has got two different levels.
(Refer Slide Time: 10:28)
Here you have two data levels and two signal levels. This is an example.
And at the bottom as you can see you have two data levels 1 0 1 0 so it is binary data but
it has got three signal levels plus, 0 and minus A that is plus A, 0, minus A that means
positive voltage, negative voltage and 0 voltage. So as you can see here you have got
positive A then 0 here then negative A here. These are the three levels that is being used
here for the purpose of encoding. And whenever you do multilevel encoding the
information that is contained can get changed and as we know I is equal to 2B into log2M
that means the number of signal levels that is being used will decide the information rate
where B is the bandwidth of the medium so if you get 2 Achieved higher data
information rate we can go for multilevel encoding if necessary.
Then comes the question of bit rate versus baud rate.
As I have explained in the last lecture the bit rate is essentially the data rate the rate at
which the signal is transmitted. On the other hand the baud rate is essentially the number
of signaling elements. So the number of signaling elements used can be more than the
data rate or can be less than the data rate. So, if you use multi-level encoding then the
baud rate will be less than the data rate. in some cases the baud rate can be more than the
data rate. Hence we have to decide on that but obviously we shall prefer lower baud rate
to achieve higher data rate of transmission through the transmission medium.
Then comes the question of DC component. As we know whenever we try to send signal
through a transmission media it is difficult to send DC signal because of the presence of
transformers, capacitor in the equipment through which we are sending the signal. So,
because of the presence of transformer and capacitor DC component the DC component
will be blocked which will lead to attenuation.
But sometimes there will be DC component. for example if it is encoded in this manner
and if you have got 0V and say another volt is plus k then as you can see this will be for
1, this is for 0 and this is even for 1 and so on so the average value will have a DC. So
that DC component will not pass through the medium. That’s why we shall try we shall
try to do the encoding in such a way that DC component is not present. That means we
shall transform the sig data into a form of signal which will not have DC component.
Then comes the question of signal spectrum.
Signal spectrum will play an important role because the bandwidth of the signal that we
are trying to send through a medium will decide whether it will be distorted or not. That
means the bandwidth has to match with the transmission media so signal spectrum has to
be taken into consideration when you do the encoding.
Noise immunity: We have to see whether the encoding that we do has higher noise
immunity or not. As we have seen some of the transmission media has got inherent
noises. We have already discussed in the transmission impairment lecture. There we have
seen that in presence of noise we have to transmit signal and obviously if the encoding is
done such that it has higher noise immunity it will be beneficial. Then we can try to do
encoding which will help us in detecting errors. Whenever the signal is sent through the
communication media because of various types of noises error is introduced in the signal.
Thus at the receiving end if you can detect error because of the encoding use that will be
helpful. So we shall look into it
Then comes the question of synchronization.
In synchronization we shall be sending signal. This is sent by the transmitter. We want
the same thing to be received at the receiving end. But if this position is not matched then
the receiver’s end must be able to identify when this is going from low to high or when it
is going from high to low.
(Refer Slide Time: 16:58)
That means these positions have to be identified at the other end. Usually a clock is used
at the transmitting end to generate the signal. Similarly at the receiving end a clock is
used to regenerate the signal. Now these two clocks may not have exactly the same
frequency so that may lead to lot of synchronization. Not only they may not have the
same frequency but their phases may also be different so it is necessary to synchronize
them. And for the purpose of synchronization there are 2 alternatives. What can be done
is a separate line can be used, a separate channel can be used for sending the clock to the
receiver. The receiver can use that clock for receiving the data but that is not really
feasible because of high cost. So commonly it is necessary to regenerate clock from the
received data with the help of some special hardware such as phase lock loop.
However, this regeneration of clock at the receiving end is possible only if when the
signal has got sufficient number of transitions. That means suppose you are sending a
long sequence of 0 or a long sequence of 1 if the encoded signal does not have enough
number of transitions then the phase lock loop will not be able to regenerate the clock so
it is necessary to have enough number of transitions in the signal for the purpose of
synchronization. Therefore while doing encoding we have to take into consideration this
synchronization aspect. Finally the cost of implementation we have to see because
ultimately you have to encode the signal with minimum cost. So these are the important
characteristics you have to look into for encoding.
Now let us see what are the various line coding schemes that is commonly used. The line
coding techniques can be broadly divided into three types; unipolar, polar and bipolar. So
these are the three basic schemes that is being used.
(Refer Slide Time: 17:32)
The unipolar is the simplest and here you have got the two voltage level. Uni means 1 so
you are sending only voltage of one polarity say 2A.
(Refer Slide Time: 18:23)
And here you are not sending anything that is considered to be 0V. That means when you
are not sending anything that is 0V when you are sending something that is 2A volt. So
only one polarity signal you are sending and when you are not sending that is been
considered as 0V. That’s why although you are sending one polarity we may consider it
as two voltage levels. So don’t get confused whenever we say unipolar at the same time
two voltage level. Essentially we are sending voltage of one polarity but whenever the
volt signal is not present then we consider it 0.
For example, for 1 we are sending voltage of 2A and when it is 0 we don’t send anything.
Similarly for 2A again we send 2A voltage and for 0 don’t send anything. That means the
data is represented by voltage level. That means one is voltage level 2A and 0 is no
voltage or no signal is being sent. That’s how this unipolar encoding is done.
Unipolar encoding has got the following characteristic. It uses only one polarity of
voltage level as I mentioned and here the bit rate is same as the data rate. The means the
modulation rate or baud rate is same as the data rate. In other words you have got only
one signaling element per bit. Unfortunately as you have seen we are using signal of only
one polarity and as a consequence there will be DC component of the average value of
the signal that will be received at the other end will have DC so there will be DC
component present in signal.
(Refer Slide Time: 19:14)
And as you can see from this signal whenever you have got long sequences of 0s or long
sequences of 1s only one signal level is transmitted or no signal is transmitted. In other
words there will be no transitions and that will lead to loss of synchronization for long
sequences of 0s and 1s. So this unipolar scheme is possibly the simplest but it is obsolete.
It is not used because of the limitation.
For example, the DC component is present which is a bad feature, loss of synchronization
is a bad feature and of course data bit rate and data rate is same. We may come we may
not consider it go good but it is not definitely bad. But it is simple because it uses only
one polarity of signal. Let us see what are the polar schemes used.
(Refer Slide Time: 20:39)
In case of polar essentially two voltage levels are used; one is positive and the other one
is negative. So we are using one positive voltage and one negative voltage and the polar
encoding scheme has got several alternatives like NRZ Non-Return-to-Zero, RZ Return-
to-Zero, Manchester encoding, Differential Manchester encoding. so these are the four
different encoding schemes available under polar encoding.
Let us consider their features one after the other, Non-Return-to-Zero that is NRZ. In this
particular case as you can see from this diagram voltage level is constant during a bit
interval. That means for 1 it has got two varieties NRZ L and NRZ I.
(Refer Slide Time: 22:56)
First let us consider the NRZ L. in case of NRZ L Non-Return-to-Zero L stands for low
level. Here it means that the binary value 1 is represented by low level. As you can see
here binary value one is represented by low level minus A and 0 is represented by high
level plus A. So here we are using signals of two polarity plus A and minus A and these
two polarities are use to represent a binary 0 and 1. So 1 is represented by A the minus A
polarity voltage and 0 is represented by plus A. That means the signal levels state of the
data is represented by signal level and that remains constant during a bit interval as I
mentioned. So you see here that during the inter bit interval the data is either plus A or
minus A (Refer Slide Time: 22:40). So when it is 1 it is minus A and when it is 0 it is
plus A.
Let us look at the other alternative NRZ I where I stands for inversion. Here in case NRZ
I what we are doing is for each one in the bit sequence the signal level is inverted.
(Refer Slide Time: 23:11)
Here as you can see this is your 1 for which it was minus A. whenever the next A is
encountered you can see here it is making a transition from minus A to plus A. Another
one is here so in this boundary again it is making transition from plus A to minus A. Here
there are two 0s so there is no transition. So whenever a 1 is encountered the signal is
making a transition or it is getting inverted from the previous signal level.
So here it was inverted from minus A to plus A and here it was inverted from plus A to
minus A because of the presence of this one, again another one is encountered here, here
it is inverted from minus A to plus A. This is how a transition from one voltage level to
the other voltage level takes place in this particular case. In the previous case as you can
see (refer Slide Time: 24:11) whenever there is a transition from 1 to 0 the data changes
from 1 to 0 so there is a transition or in the next case whenever consecutive 1s are there
or a 1 is encountered there is transition.
But whenever you have a long sequence of 0 then there is no transition in the signal and
that will lead to loss of synchronization. So here are the characteristics of this NRZ
encoding. Here you can see it has got two voltage levels plus A and minus A, here the bit
rate is same as the data rate because the number of signaling elements per bit is 1 so data
rate is same as the baud rate. So in other words it is M is equal to 2.
(Refer Slide Time: 24:53)
Loss of synchronization for long sequences of 1s and 0s, whenever you send long
sequence of one or long sequence of 0 you will find there will be loss of synchronization.
And in this case if you look at the signal spectrum you will find that most of the energy is
concentrated between 0 and half the bit rate as it is shown in this diagram.
(Refer Slide Time: 25:44)
Here you find for NRZ I and NRZ L the signal spectrum is shown here. On the x axis you
have got normalized frequency f by r where r is the bit rate and means square voltage per
unit bandwidth. So here you find most of the energy is around this region that is half of
the bandwidth. That means this is one, this is the bandwidth of the signal. You can see
here that most of the energy is between DC and the middle point of the signal around 0.5.
So here major part of the signal is shown and most of the energy is concentrated between
0 and half the bit rate. So this is the characteristic of NRZ encoding both NRZ L and
NRZ I.
Synchronization has been found to be a major concern in the two schemes that we have
discussed. To overcome that we can use RZ encoding that is known as Return-to-Zero.
So here what we do is for each bit it returns to 0. If it is 1 it is 1, if data is 1 it is plus A
then in the middle point of the bit it returns to 0 and it remains there and for 0 the signal
value is minus A again in the middle point it returns to 0.
(Refer Slide Time: 26:52)
So here we see for each bit there are two transitions. One is from low to high and another
is high to low. Thus two transitions per bit is taking place irrespective of whether is 0 or
1.
(Refer Slide Time: 28:51)
So if you have long sequences of 0 or long sequences of 1 you will not face any difficulty
because the number of transitions is quite large and as a result it is very good for
synchronization. However, it uses three levels of encoding so it is possible to have log23
that means the information content can be more but unfortunately you are encoding only
a two data that means the data rate is not increased although it has the capability to have
higher information rate.
So however the bit rate is double than that of data rate because the number of transitions
it is making is not at all an efficient encoding so here baud rate will be equal to two that
of data rate so this is definitely bad that the signal requires higher bandwidth than the data
rate. However, it provides you good synchronization as you have seen. And the increase
in bandwidth is the main limitation or concern because of this the signal has got double
the bandwidth than the data rate. Data rate is multiplied by two to the baud rate and as a
consequence RZ encoding is good from the view point of synchronization but it is bad
from the view point of the bandwidth requirement.
Let’s see how we can go for a better scheme. This Manchester encoding is also a bipolar
encoding. Here we are also using plus A and minus A the signals of two polarity levels.
However, as you can see we can consider it that each signal element is divided into two
phases biphase, two phases the first part and second part. So in Manchester encoding
there is a mid-bit transition so mid-bit transition is taking place.
(Refer Slide Time: 29:42)
So each mid-bit transition for example for one the transition is from minus A to plus A
for 0 it is from plus A to minus A. So, if the next bit is one then it has to go from minus A
to plus A so there is no need of transition in the middle, however, the next bit is 1 and
here it will make transition from low to high (Refer Slide Time: 30:26) and in this middle
position it has to again make transition from low to high so in the beginning it is making
transition from high to low. Hence it is possible to have low to high transition. That
means whenever you have two consecutive 1s as we can see the number of transitions
will be low to high, high to low then low to high.
On the other hand whenever you have got two consecutive 0s again there will be
transition from high to low, low to high and again from high to low. So the number of
transitions is more for consecutive 1s and consecutives 0s. However, when you have got
consecutives of 0s and 1s then as you can see the number of transitions is one per bit.
However, you have got enough number of transitions. this is Manchester encoding where
the signal levels are represented by the number of transitions, that is it is essentially it is
making from high to low or low to high in the middle bit position.
(Refer Slide Time: 32:48)
Let us see the other one the differential Manchester which is known as biphase encoding.
Here the presence of transition in the beginning of a bit position represents a 1. Here this
is the beginning of a bit position, this is the beginning of a position and this is a
beginning of a bit position (Refer Slide Time: 31:52) if it is a 0 then there is always a
transition in the beginning. For example, here there is a transition but it can be from high
to low or low to high there is no distinction. If it is 1 there is no transition so it depends
essentially on the previous bit.
For example, for one or 0 in the middle there will be always transition which is used for
synchronization purpose. That means in the middle there is always transition. So as you
can see here there is transition, here there is transition in the mid-bit position but only in
the beginning there is transition if it is a 0.
So this is a 0 bit so there is transition in the beginning, this is a 0 bit there is a transition
in the beginning, this is another 0 bit so there is transition in the beginning so for each of
this bits so there is transition. Hence there is transition here, here, here for all the 0s and
on the other hand there is transition in the middle for the purpose of synchronization. So
you can easily recover data from this and at the same time it will provide you very good
synchronization. So it has got two voltage level plus A and minus A as you have seen and
there is no DC component.
As you can see here alternately you are making it high and low and high and low so the
average value will be 0 so there will be no DC component present. It is a very good
feature and it provides you very good synchronization because number of transitions per
bit element will be at least one. There may be more than one transition in case of
consecutive 0s or consecutive 1s but it gives you at least one transition so it gives you
very good synchronization.
(Refer Slide Time: 34:13)
However, the bandwidth is increased here, it gives you higher bandwidth due to doubling
of bit rate with respect to data rate. That means when you are sending a sequence of 0s
and sequence of 1s then the bit rate is a data rate, the baud rate is double than the data
rate which is a bad feature and definitely this is not advantageous. But because of these
two features it is attractive and widely used in many practical encoding schemes.
Let us look at the bandwidth of these two signals Manchester encoding and differential
Manchester encoding with respect to NRZ I and NRZ L.
(Refer Slide Time: 35:21)
As you can see in case of NRZ I and NRZ L the bandwidth was very close to 0. On the
other hand for Manchester encoding as we can see there is no DC component, there is no
signal component closer to DC 0V and most of the NRZ is concentrated around the bit
rate that is 1.
However, it spreads further so it can have higher frequency signal components for both
the cases and it gives you some kind of bandpass which means this can be very efficiently
sent through a bandpass channel because the frequency spectrum has got higher
frequency components around the middle and on either side the frequency components
are less. So it is a very good frequency characteristic except that the bandwidth
requirement is higher.
(Refer Slide Time: 35:43)
Now let us focus on the bipolar encoding schemes. We have discussed unipolar and polar
and now you are considering on focusing on bipolar encoding. In bipolar encoding we
shall be using a technique known as AMI - Amplitude Mark Inversion. This terminology
mark has come from the day of telephony. In telephony we used to send signals in the
form of mark and space so that terminology has been borrowed here and here the AMI is
one kind of bipolar encoding. As we shall see this is known as bipolar AMI. It uses three
voltage levels such as plus A, 0 and minus A. And unlike Return-to-Zero the 0 level is
used to represent 0 here. In case of RZ encoding what we did the 0 voltage level was not
representing either 0 or 1 so signal was returning to 0 both for 0 and for 1.
(Refer Slide Time: 37:54)
But here it is not so. The 0 is represented by, that means the 0 bit is represented by 0V.
On the other hand a one can have either positive voltage or negative voltage so it
alternately changes. For example, for this it is plus A, next one will have minus A, next
one will have plus A, next one will have minus A so that’s why it is called alternating
binary 1s or having alternating positive and negative voltages. So this alternating positive
and negative voltages can be sensed to identify 1s.
However, if there is a long sequence of 0s then obviously there will be no transition so
this will again lead to loss of synchronization as we shall see.
So here there is no DC component because it uses positive and negative. However loss of
synchronization occurs for long sequence of 0s and as a consequence it gives you lesser
bandwidth.
Somewhat similar to bipolar AMI we have got pseudoternary. It is the same as AMI but
the alternating positive and negative pulses occur for binary 0 instead of binary 1. In the
previous case we have seen that alternating voltages are taking place for 1. In case of
pseudoternary if we replace the 1s by 0s and 0s by 1s it will be pseudoternary.
(Refer Slide Time: 38:28)
So the characteristic will be same as bipolar AMI and not different that means the
characteristic will be same as the bipolar AMI for pseudoternary. But the only difference
is that here binary 1 represents 0 and for 1 it is alternating positive and negative voltages.
That’s the difference between bipolar AMI and pseudoternary. So you can use either of
the two having the same characteristics. And obviously this is a very good feature, there
is no DC component, unfortunately loss of synchronization for long sequences of 0s is
not useful. However, it has a very attractive feature which is lesser bandwidth as we shall
see in the subsequent diagrams.
So before we compare the various techniques that we have discussed in this lecture. One
important parameter I have already mentioned about it has to be taken into consideration
which is the data rate and another is modulation rate.
Modulation rate is expressed in terms of baud b a u d and data rate as we know is
expressed in terms of bits per second. Now what is the relationship? The relationship
between data rate and baud rate is data rate d is equal to R by b where b is the number of
number of bits per signal element. That means a signal element can have more than one
bit, can represent more number of bits or it can have less number of bits. So depending on
that the data rate can be more or less.
(Refer Slide Time: 41:52)
For example, if the number of levels L is the number of different signal levels that is
being used then the data rate will be more than the baud rate R by b, R is the data rate that
means D is the modulation rate that means R is equal to D into log2L that means the data
rate R will be more whenever you are using more number of signal levels as we shall see.
And because of this the efficiency will be more whenever your data rate is more than the
baud rate.
Let us see what the situation is for the codes that we have discussed.
(Refer Slide Time: 42:08)
This is the modulation rate (Refer Slide Time: 42:00). the minimum modulation rate for
NRZ L is 0 0 0 both for NRZ L NRZ I and bipolar AMI does not change at all for some
values 0s and 1s so it is 0, for 1 0 1 it is alternating 1s and 0s, for NRZ L it is 1, for NRZ
I it is .05 and for bipolar AMI it is 1.0, for Manchester it is 1. 0, for differential
Manchester it is 1.5.
And look at the maximum, the maximum occurs in case of differential Manchester say
2.0, 2.0 both for Manchester and differential Manchester. And as you can see here from
the view point of modulation rate we find that bipolar AMI has got good characteristic on
the other hand the Manchester and differential Manchester requires higher modulation
rate compared to the other schemes and as a result they are not very attractive from this
view point.
However, we have seen that Manchester and encoding gives you very good
synchronization facility. That means whenever you are sending the signal either a long
sequence of 0s or long sequence of 1s it will not lead to synchronization failure because
at the other end the phase locked loop will be able to regenerate the signal without any
problem.
If we compare the bandwidth of the different signals we find that the NRZ L and NRZ I
has got lower bandwidth. However, because of the presence of DC component and the
low pass nature of the signal it is not attractive.
(Refer Slide Time: 44:07)
On the other hand Manchester and differential Manchester encoding has got very good
bandwidth spectrum shape. It has got most of the frequency components in the middle
however it has got higher bandwidth. The AMI and pseudoternary has got lesser
bandwidth and it has got very good frequency spectrum and most of the energy is
centered around the middle of the frequency and there is no DC component as you can
see. Hence this is the bandwidth comparison of the different encoding techniques that we
have discussed.
Of course there are other special purpose codes that is being used in digital transmission
one is known as 2B1Q, 2 binary 1 quaternary. Compared to polar or bipolar it is four
voltage levels in contrast to two voltage levels that is commonly used in polar. Here we
see for 0 0 it has got minus 3V, for 1 1 it is plus 1.5V, for 0 1 it is minus 1.5V and for 1 0
it is plus 3V. So we see here it has got four voltage levels so it gives you higher data rate.
That means R is equal to D if we look at the previous diagram this one that R is equal to
R by b that means R is equal to D into log2L.
(Refer Slide Time: 46:57)
So here R is equal to D into log2L and L here is 4. Because it is using four levels the data
rate will be two times and as a result each signal element will be able to represent two
bits. Two bits means this signal element minus 3 is representing 0 0 or this signal element
that is plus 1.5 is representing 1 1 so we see that by using this multilevel encoding we are
able to send more data with lesser number of signal elements. In other words with lesser
baud rate your data rate is more. However, because of larger number of signal levels it
will be more prone to error. So signal to noise ratio of the medium has to be better only
then this can be used.
Then there is another special purpose code that is used known as MTL-3 Multiline
Transmission. Here also we use three levels it is very similar to NRZ I. in case of NRZ I
we have seen that we are using inversions Non-Return-to-Zero inversions but instead of
two levels here we are using three levels.
(Refer Slide Time: 47:31)
Here we see that signal transition occurs at the beginning of each one bit so whenever it
is a one it is making a transition from 0 to 1 and here also it is making a transition from 1
to 0 so here we can see that we are using say 0 to plus 1 then plus 1 to 0 again from 0 to
minus 1 so in this way it is using three voltage levels and signal transition is occurring at
the beginning of each one. So it is very similar to NRZ I but uses three different levels
that is the basic difference that is used in the beginning of each one bit that is the
characteristic of this code.
Now we have seen that bipolar AMI has got very good characteristics but unfortunately it
has got bad signal characteristic so the bandwidth characteristic is bad which can be
improved by a technique known as scrambling scheme. This can be considered as
extension of bipolar AMI. Particularly when we are sending signal over a long distance
the bandwidth of the channel is very precise.
(Refer Slide Time: 50:04)
When it is for short distance communication like local area network then bandwidth is
not really very important. On the other hand whenever we are sending over a long
distance bandwidth has to be very judiciously utilized. In such cases we shall go for this
kind of special schemes and try to develop better utilization of the bandwidth. And some
of the characteristics of these special encoding codes are no DC component no long
sequence of 0 level, line signal and so on. That means whenever there is long sequences
of 0 level we shall try to enforcibly introduce some transition with minimum increase in
bandwidth as we shall see in the next lecture.
Then it should have some error detection capability. Examples of such scrambling
schemes are B8ZS and HDB3 which we shall discuss in the next lecture. So in today’s
lecture we have discussed about several schemes like NRZ, RZ and bipolar schemes and
here are some of the review questions.
1) Why do you need encoding of data before sending over a medium?
2) What are the four possible encoding techniques give examples?
3) Between RZ and NRZ encoding techniques which requires higher bandwidth and
why?
(Refer Slide Time: 50:55)
4) How Manchester encoding helps in achieving better synchronization?
Here are the answers to the questions of lecture-5.
On what parameters the quality of transmission depends in case of transmission through
unguided media?
(Refer Slide Time: 51:12)
Answer is: In case of unguided media the transmission quality primarily depends on the
size and shape of the antenna and the bandwidth of the signal transmitter.
So we have already discussed in detail the unguided media. There we have seen the size
and shape of the antenna and the bandwidth of the signal that we are sending will play
important role.
2) What are the factors responsible for attenuation in case of terrestrial microwave
communication?
(Refer Slide Time: 51:46)
We have seen that the attenuation characteristic is represented by the formula 10 log 4 pi
d by lambda square. So it depends on the distance and the attenuation increases at the rate
of distance square. However, the attenuation also increases with frequency or inversely
proportional to lambda square. Moreover during rainfall attenuation is less if there is no
rain. So in other words when there is rain attenuation is more.
3) What parameters decide the spacing of repeaters in case of terrestrial microwave
communication?
The parameters are the height of the antenna h and the adjustment factor k based on this
relationship 7.14 root kh where d is the distance in kilometer between the two antennas.
And based on the height of the antenna used one can calculate the distance and usually
the value of k is equal to 4 by 3 that can be used to compute the distance.
4) Why two separate frequencies are used for uplink and downlink transmission in case
of satellite communication
As I mentioned in the lecture two separate frequencies are used so that one cannot
interfere with the other and full duplex communication is possible. If we use same
frequencies then there will be interference and we cannot have full duplex
communication.
(Refer Slide Time: 52:53)
So to achieve full duplex communication two separate frequencies are used.
5) Why uplink frequencies are higher than the downlink frequencies in case of satellite
communication?
(Refer Slide Time: 53:44)
The reason I mentioned was the satellite gets power from solar cell so the transmitter is
not being of higher power because it is taking power from the solar cell. On the other
hand the ground station can have much higher power as we want less attenuation and data
signal to noise ratio, lower frequencies are more suitable for downlink and higher
frequency is commonly used for uplink. So this is the answer to the question 5 that was
the last question, thank you.
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